Thermal accommodation in nanoporous silica for vacuum insulation panels

Abstract

The thermal accommodation coefficient is a measure for the quality of thermal energy exchange between gas molecules and a solid surface. It is an important parameter to describe heat flow in rarefied gases, for example, in aerospace or vacuum technology. As special application, it plays a decisive role for the thermal transport theory in silica filled vacuum insulation panels. So far, no values have been available for the material pairings of silica and various gases. For that reason, this paper presents thermal conductivity measurements under different gas-pressure conditions for precipitated and fumed silica in combination with the following gases: helium, air, argon, carbon dioxide (CO2), sulfur dioxide (SO2), krypton, and sulfur hexafluoride (SF6). Additionally, a calculation method for determining thermal accommodation coefficients from the thermal conductivity curves in combination with the pore size distribution of silica determined by mercury intrusion porosimetry is introduced. The results are compared with existing models.

Keywords

• Accommodation coefficient
• Thermal conductivity
• Coupling effect
• Vacuum insulation
• Mercury intrusion porosimetry
• Gas influence